Seamless pipes are produced by conducting piercing rolling, elongating rolling using a mandrel bar, for example mandrel mill rolling, and sizing rolling, for example stretch reducer rolling and, further, subjecting the thus-obtained pipes, as mother pipes, to cold working, if necessary, generally in the manner described below. In the following, such production process is explained in connection with the case of applying mandrel mill rolling as elongating rolling and stretch reducer rolling as sizing rolling.
A round steel block (billet) is heated to a predetermined temperature (generally 1150-1250° C.) using a heating furnace, such as a rotary hearth type, and this billet is passed through an inclined roll type piercing/rolling machine for making a hollow shell. Then, a mandrel bar coated with a lubricant is inserted into the hollow shell and the hollow shell is passed, in the one pass manner, through a mandrel mill composed of 7 to 9 stands for roughening rolling to give a blank pipe having a predetermined size for finishing rolling, i.e. a finishing rolling blank pipe.
After this roughening rolling, the finishing rolling blank pipe is fed to a reheating furnace and reheated (generally to 900-1000° C.), the pipe outer surface alone is descaled by injecting high-pressure water jet, and the blank pipe is submitted to a stretch reducer rolling machine. Further, according to need, the pipe obtained by stretch reducer rolling is used as a mother pipe to be cold-worked and subjected to drawing working using a drawing machine or to cold working by cold rolling using a caliber roller such as a Pilger mill rolling machine to give the product seamless pipe.
On the occasion of the above-mentioned hot rolling of seamless pipes, the mandrel bar to be used in the step of roughening rolling on a mandrel mill is inserted into the hollow shell in a high-temperature condition (generally 1100-1200° C.) and thus exposed to a condition readily causing seizure by the hollow shell. The pipe profile and wall thickness after mandrel mill rolling is influenced by the roll revolving speed and roll caliber profile in the rolling step and further by the friction between the mandrel bar and the hollow shell. Therefore, for preventing the seizure of the mandrel by the hollow shell and for making the friction with the hollow shell proper so as to obtain the desired pipe profile and wall thickness, a lubricant is applied to the outer surface of the mandrel bar.
Known as such lubricant is, for example, a water-soluble lubricant based on graphite, which is inexpensive and has very good lubricating properties, as described in Japanese Patent Publication No. 59-37317, and this graphite-based lubricant has so far been used frequently. However, especially when the raw material is a stainless steel containing 10-30% by mass of Cr and roughening rolling is carried out using a mandrel bar coated with a graphite-based lubricant, the phenomenon of carburization occurs during rolling and a carburized layer having a higher carbon concentration than the carbon content in the substrate material is formed on the pipe inner surface side.
The main cause of the formation of a carburized layer in the pipe inner surface is the ingress of CO gas into steel matrix, the CO gas being formed from a part of graphite that is the main component of the inner surface lubricant, as well as from a part of carbon in the organic binder used therein, during mandrel mill rolling. As a result, the carbon concentration in a portion ranging from the inner surface to about 0.5 mm deep therefrom in a thickness-wise direction sometimes becomes higher by about 0.1% by mass than the carbon content in the base material, so that it may surpass the upper C content limit prescribed in Standard or the like in some cases.
In the carburized layer remaining at a level exceeding the prescribed limit, Cr, which is the main component forming a passivation film, namely an anticorrosive film, in stainless steel, is immobilized in the form of carbides, so that the corrosion resistance of the pipe inner surface is markedly deteriorated.
Therefore, those seamless stainless steel pipes which have allowed the formation of a carburized layer in the pipe inner surface cannot be shipped as products in as-is condition, so that means for causing the carburized layer to disappear are taken. For example, the pipe inner surface where a carburized layer remains is wholly polished or, in Japanese Patent Application Publication No. 09-201604, a special heat treatment method is proposed which comprises subjecting the pipe after finishing rolling to descaling so as to reduce the thickness of the oxidized scale layer in the pipe inner surface and then maintaining for 3-20 minutes the same in an oxidizing atmosphere at 1050-1250° C. for decarburization. However, these methods of causing the carburized layer portion to disappear have a problem in that a number of steps and considerable cost are required for the treatment.
Further, in Japanese Patent Application Publication No. 08-90043, it is proposed that, in the reheating treatment of the finishing rolling blank pipe, the blank pipe inside be filled with a gaseous atmosphere containing steam in an amount of not less than 10% by volume, followed by 2-10 minutes of heating at 980-1080° C. And, in the Example section, it is described that when the steam content is within the range of 0-9%, cracking tends to occur in corrosion testing. However, the production process according to Japanese Patent Application Publication No. 08-090043 requires a fairly large-scale steam production apparatus for continuously supplying steam in an amount of 10% or more through the pipe inside; this is not suited for mass production. Further, it becomes necessary to conduct solution heat treatment for decarburization after finishing rolling.
Further, Japanese Patent Application Publication No. 04-168221 proposes a process for producing austenitic stainless steel pipes which comprises subjecting a finishing rolling blank pipe, which is obtained by mandrel rolling using a graphite-based lubricant, to finishing rolling after 10-30 minutes of retention thereof in an atmosphere having an oxygen concentration of 6-15% within the temperature range of 950-1200° C. However, the production process proposed in Japanese Patent Application Publication No. 04-168221 is impracticable from the yield viewpoint since the scale loss is great due to a long period of time required for heating the finishing rolling blank pipe.
Therefore, recently, positive efforts have been made for the development of graphite-free lubricants and methods of using the same, instead of the above graphite-based lubricant, and Japanese Patent Application Publication No. 09-78080, for instance, discloses a lubricant which comprises, as main ingredients, layered oxides, namely mica, and a borate salt and is completely free of carbon or, if any, contains only the carbon in an organic binder component and thus has a carbon content lowered as far as possible. The method of applying this graphite-free lubricant is the same as in the case of graphite-based lubricants, and the composition of the lubricant is designed so that the lubricant performance thereof may be equal to that of graphite-based lubricants.
Since, however, such a graphite-free lubricant as disclosed in Japanese Patent Application Publication No. 09-078080 is expensive as compared with graphite-based lubricants, it is not applied, for economic reasons, in rolling such materials that do not require any consideration of the problem of carburization layer formation in the pipe inner surface. In most of the recent product sector where seamless steel pipes are demanded, there is no need of consideration of the inner surface carburization and, therefore, in elongating rolling using a mandrel bar, for example in mandrel mill rolling, graphite-based lubricants are generally used from the economic viewpoint.
In the case of producing low-carbon stainless steel pipes, however, it is necessary to take the problem of inner surface carburization into consideration. In such a case, if the same mandrel bar as the one already used in elongating rolling of most other steels is used, graphite always remains on and is adhering to the surface of that mandrel bar even when a graphite-free lubricant is used only in the production of low-carbon stainless steel pipes.
The graphite applied to the mandrel bar surface in elongating rolling of carbon steel pipes or low alloy steel pipes is spread abundantly on the mandrel bar transfer line, in particular the transfer line between the area of lubricant application and the area of mandrel bar insertion into the hollow shell. Since, however, an unexpectedly large-scale apparatus is required for washing the production line, no sufficient washing is generally done and the contamination with graphite from the production line is inevitable.
Therefore, even when a graphite-free lubricant is applied to the surface of the mandrel bar for using the same in elongating rolling of low-carbon stainless steel pipes, the surface thereof (namely, the surface of the graphite-free lubricant film) is partly contaminated with the graphite already spread on the transfer line, irrespective of whether the mandrel bar has been submitted to elongating rolling with a graphite-based lubricant applied thereto or not.
This graphite partly adhering to the graphite-free lubricant film surface comes into direct contact with the workpiece, namely the hollow shell; this causes the formation of a partially carburized layer in the pipe inner surface after rolling. Thus, the formation of a carburized layer is caused although there is a difference in extent as compared with the case of using a graphite-based lubricant.
On the other hand, in cases where a mandrel bar already submitted to elongating rolling with a graphite-based lubricant applied thereto is used, graphite remains adhering thereto beneath the graphite-free lubricant film newly applied and, as a result of severe working on an elongating rolling mill, the graphite remaining beneath the film also comes into direct contact with the workpiece and causes the formation of a partial carburized layer in the pipe inner surface during rolling and in the subsequent steps.
In this way, even when a graphite-free lubricant is used in elongating rolling using a mandrel bar, a carburized layer is formed in the pipe inner surface and causes deterioration in corrosion resistance.